The present invention relates generally to the field of digital radio communication systems and, more particularly, to the periodic registration procedure in a digital radio communication system used to identify and update the location and activity status of mobile radio communication devices within said system.
In a cellular communication network, the geographic area covered by the network is divided into a plurality of cells. Each cell has a base station for communication with the mobile stations within that cell. The base station serves as an interface between the mobile stations in the cell and the network. To maintain communications with the mobile station, the network needs to track the location of the mobile station as it travels from one cell in the network to another cell. Tracking the location of the mobile station is accomplished by requiring the mobile station to periodically register with the network. During the registration procedure, the mobile station sends a registration message to the network. The network notes the location of the mobile station that is recorded in the network""s home location register (HLR) or visitor location register (VLR).
The HLR includes a database for storing subscriber information such as the subscriber""s international mobile subscriber identification (IMSI), the subscriber""s phone number from the public network, the subscriber""s permitted supplementary services, and information related to the subscriber""s current location. The HLR provides the MSC with the necessary subscriber data when a call originates from the public network. Much of the same information is also stored in the VLR. The VLR, however, stores more detailed information about the current location of the mobile station than the HLR. The VLR provides the MSC with the necessary subscriber data when a call originates from the mobile station.
When a mobile station is active (i.e., powered on), it periodically sends a registration message to the network which notes the location of the mobile station and updates the information in the HLR and VLR concerning the mobile station""s location and activity status. When the network fails to receive a registration message from the mobile station after a long period of time, the network may assume that the mobile station is out of service, either because the mobile station is turned off or because it is no longer present in the network. In such cases, the network may update the HLR and VLR to indicate that the mobile station is inactive. Thereafter, if a call is addressed to the inactive mobile station, the network, instead of paging the mobile station, will simply send a message to the caller advising the caller that the mobile station is out of service.
IS-136 is a communication protocol for a TDMA wireless communication system that employs a digital control channel (DCCH). IS-136 is published by the Telecommunications Industry Association. Mobile phones implementing this protocol are required to register periodically on a DCCH. After sending a registration message to the base station, the mobile station enters a registration proceeding state and waits for a response from the base station. If the base station does not respond within a predetermined period of time, the mobile station resends the registration message up to a maximum of five times. After six registration failures, the mobile station will return to camping on the last used DCCH.
One shortcoming with the registration method described in IS-136 is that the mobile station remains camped on an obviously dysfunctional control channel. Since the base station does not respond to the registration messages from the mobile station, it is also unlikely to respond to any attempts by the mobile station to originate a call. Moreover, if the mobile station has failed at six consecutive registration attempts, the network may modify its HLR and VLR to indicate that the mobile station is inactive. Any person attempting to call the mobile station would likely receive a message indicating that it is no longer in service. Also, messages such as short text messages, message waiting indications, etc., would not be forwarded to the mobile station.
The present invention relates to registration procedures used by a mobile station which is locked onto a D-AMPS digital control channel (DCCH). In particular, the present invention relates to a method for re-establishing service on a digital control channel after a predetermined number of registration attempts have failed.
The term registration refers to the steps taken by a mobile station to identify itself to a base station as being active in the system. To register with the network, the mobile station sends a registration message to the base station and then enters the registration proceeding state. Upon entering the registration proceeding state, the mobile station starts a registration timer. If a response is received from the base station prior to the expiration of the timer, the mobile station takes action according to the response and it returns to camping on the current DCCH. On the other hand, if the registration timer expires without a response from the base station, the mobile station sets a re-registration timer and returns to camping on the DCCH. When the re-registration timer expires, the mobile station re-attempts registration on the current DCCH. After six consecutive attempts at registration have been made without a response from the base station, the mobile station assumes that the current DCCH is dysfunctional and searches for a new control channel on which to acquire service. If a suitable control channel is found, the mobile station will attempt to register on the new control channel. Thus, the mobile phone is prevented from camping on a dysfunctional control channel.
The present invention can also be applied to any DCCH scanning and locking algorithm. After a predetermined number of attempts to register have been made without a response from the base station, the mobile station would leave the current DCCH and continue the scan for another control channel.